This invention relates to catalyst substrates and more particularly to an alumina-silica catalyst substrate characterized by high strength, high relative surface area and good thermal stability.
A need exists for a catalyst support having not only the porosity characteristics necessary for proper catalyst loading but good structural properties as well. Such structural properties include resistance to high temperature and thermal shock. Preferably, the material comprising the support should be extrudable, in the green state, into complex shapes when desired to provide, for example, higher surface contact area.
For example, the anti-polution standards proposed for the reduction of hydrocarbon emissions from internal combustion engines have created a need for a catalyst substrate having a pore structure and surface area such that a catalyst structure having a high degree of catalytic activity with a minimum amount of active catalyst material and at the same time exhibiting structural strength and resistance to high temperatures and thermal shock can be produced.
It has been proposed to provide such a structure using non-porous ceramics such as cordierite which, after extrusion and firing, are coated with activatable alumina to provide porosity for subsequent catalyst loading.
However, the added step required to coat the fired non-porous substrate is costly. Furthermore, it is desirable to provide a material having higher porosity and temperature resistance.
It has also been previously proposed to use alumina in combination with silica in the form of finely divided particles such as silica gel or the like to form catalyst supports, for example, in U.S. Pat. Nos. 2,307,878; 2,356,303; and 2,565,215. However, in none of these patents is it proposed to provide a catalyst support characterized by high surface area as well as good structural strength and thermal stability. There also appears to have been some recognition in the art that the addition of colloidal silica to impure forms of alumina such as bauxite can increase the hardness of the resultant product. Heinemann, in U.S. Pat. Nos. 2,563,650 and 2,603,609 indicates that colloidal silica may be added to activated bauxite to produce a product which is harder than the initial bauxite itself. However, it is not at all clear that the indicated additional hardness is attributable to the alumina and silica alone as opposed to the additional impurities contained therein. In fact, in Venable U.S. Pat. No. 2,899,323 and Baldwin U.S. Pat. No. 3,146,210 it is taught that one should use a third ingredient which respectively is either a metal oxide or aluminum nitrate to obtain an increased hardness of the resulting product.
There remains, therefore, a need to provide a porous alumina-silica catalyst substrate which exhibits relatively high surface area without sacrificing the strength and hardness which is previously indicative only apparently of the use of such materials in combination with other additives such as metal oxides or the like, or as previously mentioned, by using non-porous substrates coated with alumina.